The present invention relates to a method and apparatus for spray coating the inside of a tubular body having a seam, and particularly a can body. It relates particularly and preferably to a method and apparatus for electrostatically spraying a pulverulent or other powdered coating material by means of a spray jet directed at the seam of the body to be spray-coated.
As long as cans have been used as containers for beverages and foodstuffs, they have been made from so-called tinplate, i.e. steel plate covered with a thin layer of tin. Since it has been realized that direct contact between the food and the tin is undesirable, the inside of tinplate cans is normally sprayed with a thin lacquer. Today so-called three-part cans are predominantly used for foods and beverages. A three-part can consists of a body, constituting the usually cylindrical side walls of the can, a bottom and a top. The can bodies are made from rolled metal plate. After a rectangular piece of plate that is to be made into a can body is rolled, the pair of now adjacent opposite edges are connected by a double-seam or a special welding. The seam or welding requires a thicker coating than the rest of the can body. The reasons for this have been described in detail in German Utility Model No. 76 36 666. The top and bottom of the can are then joined to the can body.
Two approaches have been used in the past to solve the related problems of coating an article which has different bonding strengths for the coating material at different places, and of coating an article with different thicknesses of coating material placed at different places. Either the areas that must be differently coated are coated in separate operations, or else the entire surface of the article must be coated with a uniform thickness at least as great as the maximum thickness of coating required anywhere on the surface of the article. The former method, which involves a separate coating operation for each area that must be coated to a different thickness, employs very expensive spraying apparatus and requires a great amount of time, due to the multiple spraying operations. The latter method is faster and can be done with less expensive equipment but requires a large amount of coating material. In order to minimize total costs, the second approach has been preferred in the past, despite the consumption of excess coating material. Since, however, the coating material is also expensive, it has been proposed in German Utility Model No. 76 36 666, mentioned above, to coat the seams of the can body seams by means of a guide body which concentrates the coating material onto the seams. The remaining portion of the interior of the can is sprayed before or after the seams are sprayed. Thus two operations are employed in order to save coating material.
U.S. Pat. No. 3,081,947 discloses a device for spray coating the interior of a can with a liquid. According to this known method, the spraying apparatus and the can body must be turned relative to each other during the spraying in order to coat all regions of the interior of the can. This procedure requires that the coating material be fed to the spraying apparatus discontinuously. The apparatus used is expensive and prone to breakdowns.